Niobium-titanium refractory alloy



United States Patent 3,001,870 NIOBIUM-TITANIUM REFRACTORY ALLOY Douglas G. McCullough, Rochester, and Neil M. Lottridge, Jr., Warren, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Filed Jan. 15, 1960, Ser. No. 2,601 3 Claims. (Cl. 75-174) This invention relates to a niobium-titanium alloy having a high streng-th-to-weight ratio together with excellent oxidation resistance, fabricability and ductility. It pertains particularly to an alloy of this type which is useful as a skin or casing material for aircraft, missiles and rockets or for other applications where a combination of oxidation resistance, high strength and low weight is important.

Today there are commercially available refractory alloys, such are nickel base alloys and cobalt base alloys, which have reasonable strength and fairly good oxidation resistance at elevated temperatures. These alloys are useful as turbine bucket or nozzle guide vane materials. However, as is true with stainless steel, the strength-toweight ratio of these alloys is relatively low, and hence they cannot be used to full advantage for rocket casings, aircraft skins and similar applications.

Accordingly, a principal object of the present invention is to provide a refractory alloy which can be employed as a skin or shell material for missiles or aircraft or for casings of solid propellant rockets or the like. This object is attained in accordance with the present invention with a niobium-titanium alloy comprising about 35% to 45% titanium, 9% to 12% chromium, 2% to 8% aluminum, 0.5% to 2% manganese and the balance substantially all niobium. This alloy is very easily fabricated and has excellent ductility, both at room temperature and at elevated temperatures. The melting point of this alloy is in excess of 3500 F. Optimum results are obtainable when the alloy contains approximately 38% to 50% niobium, 38% to 42% titanium, 9% to 11% chromium and 3% to 7% aluminum.

Small amounts of various other elements, usually less than about 2% or 3%, can be present in the niobiumtitanium alloy without detracting from its desirable physical properties. For example, a small quantity of carbon, not in excess of about 0.25% can be tolerated. The alloy becomes diflicult to fabricate if it has a higher carbon content.

The titanium greatly improves the high temperature oxidation resistance of the alloy, although it is also necessary to provide the alloy with the desired amount of ductility. If the titanium content is less than about 35%, the oxidation resistance of the alloy at elevated temperatures is inadequate and the alloy tends to be brittle. On the other hand, when more than about 45 titanium is present, the strength (particularly the hot strength) of the alloy is reduced to an excessive extent.

Chromium also contributes to the oxidation resistance of the niobium-titanium alloy as well as somewhat increasing its hot strength. A chromium content of at least 9% is necessary for outstanding oxidation resistance. However, if more than about 12% chromium is present, the alloy becomes brittle and cannot be properly fabricated by rolling or the like. The optimum combination of physical properties appears to be achieved with about 9% to 11% chromium.

Patente Sept. 26, 196.1

As hereinbefore indicated, approximately 2% to 8% aluminum is included in the alloy, and this element further improves the oxidation resistance of the base material. If the aluminum content is raised above this level, however, the ductility of the alloy is decreased to too great an extent. This reduction in ductility appears to be the result of the excess aluminum promoting the formation of brittle intermetallic compounds. An aluminum content of less than 2% does not appear to materially increase the oxidation resistance of the niobium-titanium alloy.

The manganese in the alloy serves to improve its high temperature oxidation resistance by causing a protective oxide scale to be formed on the surfaces of the alloy. In general, at least 0.5% manganese is need to provide sheet material with an adequate layer of this scale. Manganese in excess of about 2% reacts with the titanium to form a low melting phase which causes hot shortness and brittleness of the alloy.

The above-described niobium-titanium alloy has been prepared by alloying high purity elemental raw materials in an inert atmosphere of argon plus helium. It was found that the constituents of the alloy may be added either simultaneously or successively. Melting was accomplished with a non-consumable tungsten electrode electric arc. After solidification of the alloy, it was crushed to fine particle size and re-melted in the foregoing manner and cast in ingot form. These ingots were then annealed under vacuum at a temperature of about 2000 F. to 3000 F. and subsequently worked by swaging and/ or rolling. For example, a sheet having a thickness of 0.01 inch with approximately cold work was produced by rolling an ingot at room temperature.

The cold rolled alloy sheet thus produced was subjected to a number of tests to determine various physical properties. For example, an alloy consisting of about 44% niobium, 40% titanium, 10% chromium, 5% aluminum and 1% manganese was found to have an ultimate tensile strength of approximately 259,000 p.s.i., a yield strength of about 250,000 p.s.i., a density of only 5.5 grams per cc. and bend ductility of more than 180 around a bar having twice the thickness of the test sheet. This combination of physical properties is not possessed by any of the various nickel base alloys, cobalt base alloys and titanium base alloys heretofore developed.

We have also evaluated the alloy of this invention from the standpoint of oxidation resistance. For example, an alloy having the foregoing composition had a total surface metal loss of approximately one mil in thickness after hours cyclic exposure in air at a temperature of 2000 F. By comparison, pure niobium undergoes a surface metal loss of about one-quarter inch when subjected to the same test, thus demonstrating the greatly improved ance and a high strength-to-weight ratio, said alloy con- References Cited in the file of this patent sisting essentially of about 38% to 50% niobium, 38% to 42% titanium, 9% to 11% chromium, 3% to 7% UNITED STATES PATENTS aluminum and 0.5% to 2% manganese. 2,754,204 Jaifee et a1. July 10, 1956 3. A niobium-titanium alloy having an outstanding 5 2,819,960 Bomberger Jam 14, 1958 combination of hot and cold ductility, fabricability, oxi- 2,822,268 Hix Feb. 4, 1958 datlon resistance and strength-to-welght ratio, said alloy FOREIGN PATENTS consisting of approximately 44% niobium, 40% titanium, 10% chromium, 5% aluminum and 1% manganese. 718,882 Germany Mar. 24, 1942 

1. A DUCTILE, OXIDATION-RESISTANT ALLOY HAVING A HIGH STRENGTH-TO-WEIGHT RATIO, SAID ALLOY COMPRISING ABOUT 35% TO 45% TITANIUM, 9% TO 12% CHROMIUM, 2% TO 8% ALUMINUM, 0.5% TO 2% MANGANESE AND THE BALANCE SUBSTANTIALLY ALL NIOBIUM. 